1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, more particularly, an LCD module using a light emitting diode (LED).
2. Discussion of the Related Art
Flat panel display (FPD) devices that have a relatively light weight, a thin profile, and low power consumption characteristics are being developed and commonly used as a substitute for cathode ray tube (CRT) devices. Generally, display devices may be classified according to their ability for self-emission, and may include emissive display devices and non-emissive display devices. Emissive display devices display images by taking advantage of their ability to self-emit light, while the non-emissive display devices require a light source since they do not emit light by themselves. For example, plasma display panel (PDP) devices, field emission display (FED) devices, and electroluminescent display (ELD) devices are commonly used as emissive display devices. Liquid crystal display (LCD) devices may be categorized as non-emissive display devices commonly used in notebook and desktop computers because of their high resolution, capability of displaying colored images, and high quality image display.
An LCD module of the LCD devices include an LCD panel for displaying images to an exterior and a backlight unit for supplying light to the LCD panel. The LCD panel includes two substrates facing and spaced apart from each other, and a liquid crystal material interposed therebetween. Liquid crystal molecules of the liquid crystal material have a dielectric constant and refractive index anisotropic characteristics due to their long, thin shape. In addition, two electric field generating electrodes are formed on the two substrates, respectively. Accordingly, an orientation alignment of the liquid crystal molecules can be controlled by supplying a voltage to the two electrodes, wherein transmittance of the LCD panel is changed according to polarization properties of the liquid crystal material. However, since the LCD panel is a non-emissive-type display device, an additional light source is required. Thus, a backlight unit is disposed under the LCD panel, wherein the LCD device displays images using light produced by the backlight unit. In general, backlight units may be classified into two types according to a disposition of the light source, such as side-type backlight unit and direct-type backlight unit. As display areas of the LCD devices become larger, direct-type backlight units including a plurality of light sources have become more commonly used in order to provide increased brightness.
Generally, discharge lamps, such as a cold cathode fluorescent lamps (CCFL) or external electrode fluorescent lamps (EEFL), are used as a light source of the backlight unit. Additionally, a light emitting diode (LED) are gradually being used more often as the light source of the backlight unit to improve color reproducibility to increase brightness of the display.
FIG. 1 is an assembly view of an LCD module using an LED according to the related art. An LCD panel 10 and a backlight unit 20 are assembled with mechanical structural elements to protect these components and to prevent light leakage from the assembled LCD module 1. Generally, the LCD panel 10, the backlight unit 20, and several mechanical elements may be totally referred an LCD module 1.
The LCD module 1 includes an LCD panel 10, a backlight unit 20 located under the LCD panel 10, a main support bracket 40, which is preferably square or rectangular, surrounds the edges of the LCD panel 10 and the backlight unit 20. Additionally, a bottom cover 50 is provided on a bottom surface of the backlight unit 20 to provide structural support and to prevent light leakage from the unit when which is combined with the main support bracket 40. Furthermore, a top cover 60 surrounds a front edge of the LCD panel 10, the LCD module 1 is totally assembled and combined by the main support bracket 40, the bottom cover 50, and the top cover 60.
Gate and data driving circuit boards 14, 16 are disposed along at least one edge of the LCD panel 10, which generate signal voltages for controlling an image on the LCD panel 10. In some embodiments, the gate and data driving circuit boards 14, 16 are bent toward the backside of the bottom cover 50.
In addition, the backlight unit 20 further includes a plurality of printed circuit boards 22 are disposed along an inner surface of the bottom cover 50, a plurality of light emitting diodes (LEDs) 24 packaged on the printed circuit boards 22, a reflective sheet 28 having a plurality of open portions 30 corresponding to the plurality of LEDs 24 and contacting the printed circuit boards 22 and the bottom cover 50 except the LEDs 24, and a plurality of optical sheets 32 covering the plurality of LEDs 24 with the reflective sheet 28 therebetween.
Accordingly, light from the plurality of LEDs 24 is reflected directly or by the reflective sheet 28, and then the reflected light is altered during passing the plurality of optical sheets 32. The altered light is transmitted to the LCD panel 10, which allows the LCD panel 10 to display a bright image.
However, this LCD module 1 according to the related art has a several problems, including the relatively large required thickness of the unit, which prevents the unit from being as thin as possible.
FIG. 2 is a schematic cross sectional view taken along a line II-II of FIG. 1 according to the related art. A plurality of LEDs 24 are packaged with the printed circuit board 22. The printed circuit board 22 is disposed on an inner surface of a bottom cover 50 and reflective sheet 28 having a plurality of open portions 30 covers the printed circuit board 22 and the bottom cover 50 except for the LEDs 24. Each of the LEDs 24 are exposed by each of the respective open portions 30. Furthermore, a plurality of open portions 30 are disposed on the reflective sheet 28.
In the related art, a metal core type printed circuit board are often used as printed circuit boards 22. Metal core type printed circuit boards often emit significant amount of heat because the inner temperature of the LED increases when brightness of the LED is increased. Although the LED emits more heat, the internal temperature also increases which reduces brightness of the LED.
FIG. 3 is a graphic view illustrating a brightness relationship between a junction temperature and brightness of an LED according to the related art.
As discussed above, the greater the junction temperature of the LED is increased, the more the output brightness is reduced. For example, when the junction temperature is more than 80 degrees Celsius (° C.), the brightness is reduced less to 80 percent. Accordingly, the printed circuit board may be manufactured with an Aluminum core (Al), which is a material relatively a high thermal conductivity, and a relatively low resistance. The plurality of LEDs are preferably positioned along one edge of the printed circuit board, and another edge of the printed circuit board is closely adhered to the inner flat surface of the bottom cover using a conductive adhesives or a screw.
As a result, the printed circuit board except the LED and the reflective sheet covering a portion of the bottom cover have a first distance A with the inner surface of the bottom cover, wherein the first space A corresponds to a height of the printed circuit board and the conductive adhesives.
Additionally, the plurality of optical sheets is interposed between the reflective sheet and the LCD panel, wherein the optical sheets converts light emitted from the LED into a uniform plan light and transmits light to the LCD panel to improve the brightness of the panel. Specifically, the optical sheets include a diffusion sheet directly disposed on the LED and a plurality prism sheets spaced apart from the diffusion sheet.
Accordingly, a second distance K corresponds to a space from the inner surface of the bottom cover to the outer surface of the outermost prism sheet toward the LCD panel. The second distance K includes the space A between bottom cover and the inner surface of the reflective sheet facing the bottom cover and a third distance B that extends from the outer surface of the reflective sheet to the outer surface of the approximate prism sheet.
Here, the LCD module using the LED according to the related art must include the first distance A and the third distance B, so it is difficult significantly to reduce the second distance K. Furthermore, the combination process for the printed circuit board and the bottom cover using the conductive adhesives or the screw has a complicated problem.